The present invention relates to an overhead transmission line of the type provided with a communication core and to an overhead transmission line which has a data transmission line in its interior.
The operation of modern overhead transmission line networks requires fast and dependable data transmission devices in order to be able to handle the heavy and steadily increasing demand for information transmission capacity.
There are known, and still generally used, overhead transmission line networks in which the associated communication devices operate according to the TFH principle, that is with pairs of conductors.
The German symbol "TFH" (Tragerfrequenz auf Hochspannungsleitungen) corresponds to the American symbol "PLC" (power line carrier).
The data is here transmitted along a pair of overhead lines or via one overhead line with earth return. The drawback of this type of data transmission system is that only a very narrow frequency range is available, and this is even more restricted by mutual influence with adjacent overhead line networks. Another drawback in such systems is the transmission energy loss through radiation.
In order to overcome these drawbacks, separate open-air cables have been provided in the overhead transmission line networks for the transmission of data. These cables are either individually suspended in an insulating manner from all poles or towers of the transmission line, as disclosed in German Pat. No. 893,355, or are designed as coaxial cables having a high-voltage resistant insulated inner conductor and a grounded return conductor, as disclosed in German Auslegeschrift [Published Application] No. 19 33 813.
Such a separate overhead cable in an overhead transmission line network has the advantage that a broad frequency range is available for the transmission of data since there is no external influence. Moreover, it is possible to transmit data over a greater distance because no energy is lost along the transmission path. The transmission of data in overhead transmission lines via a separate overhead cable nevertheless has drawbacks since it entails additional technical efforts and costs.
It is also known to accommodate a data conductor concentrically at the interior of a hollow power conductor as disclosed in U.S. Pat. No. 1,855,288. This solution is of course limited to hollow cables, since solid cables do not have such interior space. Aside from that, such cable structure is highly expensive to produce since it is practically necessary to produce a coaxial line with an outer conductor of specially shaped wires. The laying of such phase cables requires comparatively great care and attention since there exists the danger that the hollow cables might develop kinks. Moreover, water may penetrate into the cavity and produce short circuits between the central conductor and the return conductor.
Finally the state of the art also includes a combined carrier frequency and high voltage overhead cable for the transmission of information in power transmission networks. In the center of one of the conductor cables which serves to transmit power there is a single or multiple two-wire plastic insulated carrier frequency line. Such a structure is disclosed in German Pat. No. 2,011,016. This cable has the same advantages, with respect to transmission technique, as those described above for the separate overhead cables. It has, however, the drawback that the outer diameter of the cable is greatly enlarged compared to equivalent cables because the power conductor encloses the data core so that the full power conductor cross section is added to the data core, i.e. this is an overhead communication transmission cable to which has been added the full power conductor cross section. The greater outer diameter of the power conductor results in a substantial increase in the additional loads caused by ice formation and wind forces. A further drawback is that the communication core is too delicate for the sturdy overhead cable so that great care is required in installing the cable. Moreover, the communication core itself is disproportionately heavy, resulting in asymmetrical weight loads on the poles or towers. The known structure of a combined overhead cable is thus suitable only for larger cable cross sections in which the weight and cross-sectional proportion of the data core is relatively small. Finally, the connection of cable sections by means of gripping sleeves produces difficulties.